Thousands of miles of buried natural gas pipes of varying size and formed from various materials are presently in service. All of these pipes are in some state of progressive degradation. In most instances, the extent of such degradation is unknown, and hence, the serviceability of the pipes is similarly unknown. This lack of information with the respect to the degree of degradation results in unforeseen gas pipe leaks and/or breaks, and necessitates the expending of substantial time and expense in locating these defects so that repairs and/or replacement can be made. Because of the need to detect conditions which might result in gas pipe breaks and/or leaks, an apparatus has been developed for inspecting gas pipes, and such apparatus is referred to as a pipe line "pig" or "mouse." For purposes of clarity, such apparatus are hereafter generally referred to as scan assemblies. Such pipe line scan assemblies typically include a housing with a plurality of sensors mounted to the outer surface thereof in a predetermined configuration or array to contact the inner surface of the gas pipe. Since the sensors are in a predetermined configuration or array, complete inspection of the wall of the gas pipe is generally not possible, i.e., the portion of the pipe wall between two adjacent sensors is typically not inspected. Thus, complete inspection of the wall would require an inordinate number of sensors.
In an attempt to achieve close to complete inspection, one scan assembly has incorporated rotational movement of its sensors so that, as the scan assembly advance axially down the pipe, a helical scan pattern is produced. This is accomplished by providing the scan assembly with a central, rotatable sensor section interposed two end sections which remain stationary relative to the rotating sensor section. Mounted to the sensor section are one or more sensors, such as ultrasonic transducers.
Since the aforementioned scan assembly incorporates a rotating sensor section, there must be provided means for transferring power and data across the rotating interface between the sensor section and the stationary end section(s). Two methods available for accomplishing the transfer of power and data are (1) the use of slip rings and (2) the use of low level radio frequency (RF) data transmission. Unfortunately, both of these methodologies have several disadvantages, some of which are described below. For example, slip rings introduce noise into the transmitted signal as a function of the coefficient of friction vibration. To reduce the effect of the added noise, the transmitted signals are generally passed as logic level signals, and preferably, a digitized rather than analog signal. Thus, a signal at the sensor section must be processed before being transmitted to the stationary section. This requires miniaturized electronic circuitry be mounted in the sensor section for processing of the signal so that it can be transmitted to the stationary section. The mechanical drag produced by the contact of the slip rings increases the energy lost in the motor utilized to rotate the sensor section. Further, slip rings are physically complex, and therefore, present service concerns in regard to breakage and maintenance. An RF transmitter/receiver configuration does not have the service concerns of slip rings but requires the sensor section to include more conditioning circuitry in order to multiplex the signal if more than one is to be transmitted, and to modulate the multiplexed signals onto a high frequency carder. This is more costly and consumes more power than slip ring, not to mention that power cannot be transmitted in this manner.
In addition to scan assemblies that utilize ultrasonic transducers as sensors, scan assemblies can also be equipped with other diagnostic tools such as video cameras, temperature sensors, or humidity sensors. In operation, several scan assemblies having different diagnostic tools thereon are often linked together to form an inspection train whereby power and data is passed from scan assembly to scan assembly and to above ground or "uphole" electronics units via an umbilical cord attached to the first scan assembly in the train.
In view of the foregoing, it has become desirable to develop a method and apparatus that provides an efficient, dependable and economical means for transmitting power and/or data from the rotational sensor section to the stationary section of a scan assembly.